1. Field of the Invention (Technical Field)
The present invention relates to optical detection of gases using external cavity diode lasers.
2. Description of Related Art
Diode lasers have become increasingly important for optical detection of gases. Because typical diode lasers have relatively short tuning ranges, a manner in which to widen the tuning range is to incorporate the diode laser as the gain element in an external cavity diode laser (ECDL). The implicit goal of this approach is to provide for an ECDL with narrow wavelength, often single frequency, emission. This narrow emission is then tuned through a broad wavelength range by various sophisticated mechanisms known in the art. This approach can lead to an ECDL optical source capable of high-sensitivity trace gas detection. However, it is often the case that survey spectra need to be obtained to simply determine whether there are spectral interferences between two or more species. In these instances, high sensitivity is not always required.
The most successful commercial ECDL is produced by New Focus, Inc. Their ECDLs are based on the Littman-Metcalf grazing incidence design. M. G. Littman and H. J. Metcalf, Appl. Opt. 17, 2224 (1978). The ECDL tunes single frequency through a particular mechanical movement of the cavity feedback mirror. New Focus, Inc. has expended significant effort to provide single mode tuning in a single mechanical movement. The requirement is that the cavity length change concomitantly with the angular selection of the cavity feedback mirror to provide single mode tuning. This capability is the basis of New Focus' U.S. Pat. Nos. 5,319,668 and 5,995,521. The present invention obviates the requirement for single mode tuning in by employing multi-mode operation of the ECDL.